1. Field of the Invention
The present invention relates to an antenna provided in a mobile communication terminal to transmit/receive radio signals, and more particularly, to a helical antenna installed inside a mobile communication terminal, capable of processing low-bandwidth signals.
2. Description of the Related Art
Recent trend of installing more wireless technologies in a single mobile communication terminal leads to the diversification of frequency bandwidth used by an antenna of the terminal. Particularly, frequency bandwidths currently used in a mobile communication terminal include 800 MHz to 2 GHz (for mobile phones), 2.4 GHZ to 5 GHz (for wireless LAN), 88 MHz to 108 MHz (for FM radio), 470 MHz to 770 MHz (for TV) and other bandwidths for ultra wideband (UWB), Zigbee, Digital Multimedia Broadcasting (DMB) and soon. The DMB bandwidth is divided into 2630 MHz to 2655 MHz for satellite DMB and 180 MHz to 210 MHz for terrestrial DMB.
Currently, mobile communication terminals confront demands for various service functions as well as size and weight reduction. In order to meet such demands, a mobile communication terminal tends to adopt an antenna and other components which are more compact-sized and well as multi-functional. Moreover, recent trend is that more mobile communication terminals are internally equipped with an antenna. Accordingly, an antenna to be installed inside a mobile communication terminal has to satisfy desired performance as well as occupy only a very small volume inside the terminal.
FIG. 1 is a structural diagram illustrating a general built-in Planar Inverted F Antenna (PIFA).
The PIFA is an antenna designed for installation in a mobile communication terminal. As shown in FIG. 1, the PIFA generally includes a planar radiator 2, a ground line 4 and a feeding line 5 connected with the radiator 2, and a ground plate 9. The radiator 2 is powered via the feeding line 5, and forms an impedance matching with the ground plate 9 by means of the ground line 4. In the PIFA, the width Wp of the ground line 4 and the width W of the radiator 2 should be considered in designing of the length L of the radiator and the height H of the antenna.
The PIFA has directivity. That is, when current induction to the radiator 2 generates beams, a beam flux directed toward the ground surface is re-induced to attenuate another beam flux directed toward the human body, thereby improving SAR characteristics as well as enhancing beam intensity induced to the radiator. The PIFA operates as a rectangular micro-strip antenna, in which the length of a rectangular panel-shaped radiator is reduced by half, thereby realizing a low profile structure. Furthermore, the PIFA is provided as a built-in antenna installed inside a terminal, thereby obtaining excellent endurance against external impact as well as allowing the terminal to be designed with an aesthetic appearance.
FIGS. 2 and 2a are perspective views illustrating a conventional built-in helical antenna.
Referring to FIGS. 2 and 2a, a conventional helical antenna 20 includes a feeding line 22, a ground line 23 and a helical radiator 24 formed on a dielectric substrate 21.
The feeding line 22 and the ground line 23 are formed on the underside of the dielectric substrate 21, and connected to the radiator 24. The radiator 24 includes a plurality of lower electrodes 25 formed on the underside of the substrate 21, arranged in parallel with the feeding line 22 and the ground line 23. The radiator 24 also includes a plurality of upper electrodes 26 formed on the top of the substrate 21, inclined with respect to the lower electrode 25. Each lower electrode 25 is connected at the lower end thereof with the lower end of each upper electrode 26 by means of a via 27 made of conductive paste filled into a via hole. The lower electrode 25 is connected at the upper end thereof with the upper end of an adjacent upper electrode 26 by means of a side electrode 27-1, and then with another lower electrode 25-1, thereby producing a helical antenna.
FIG. 3 is a graph illustrating resonant frequency characteristics of the helical antenna shown in FIG. 2.
FIG. 3 shows an operation frequency of an helical antenna in which a substrate 21 with a length of 20 mm, a width of 4 mm and a thickness of 1 mm was used, and the total length of the radiator 24 was 14.6 cm with 21 turns. In the graph, the horizontal axis indicates frequency (GHz) and the vertical axis indicates S11 parameter (dB). Referring to FIG. 3, it can be experimentally understood that the conventional helical antenna 20 has a resonance region 30 in vicinity of 570 MHz with radiation efficiency of 41.90%.
The conventional built-in antennas as shown in FIGS. 2 to 3 can be fabricated to have a size of about 10 mm×10 mm in a frequency bandwidth of 1 GHz or more. However, in case of a mobile communication terminal for terrestrial DMB where frequency to be processed by an antenna drops to a bandwidth of several hundred MHz or less, the antenna is required to have a length (i.e., 1/λ, ½λ or ¼λ, where λ is a wavelength of a radio-wave) that is merely several ten centimeter. Thus, conventional built-in antennas cannot process lower bandwidth frequencies of for example terrestrial DMB. Furthermore, the size of an antenna to be installed inside a mobile communication terminal such as a portable phone is limited to 5 cm or less. However, an antenna fabricated according to a conventional built-in antenna technology is sized of several ten cm or more, and thus lacks applicability as a built-in antenna.